1. Field of the Invention
The present invention relates to a traveling system transmission structure for a vehicle in which a planetary gear unit and a forward/rearward-travel switch unit are arranged in series.
2. Related Art
There has been publicly known a traveling system transmission structure including a planetary gear unit that has first and second components for receiving a constant-speed rotational power and a variable-speed rotational power respectively as well as a third component for outputting a combined rotational power of the constant-speed power and variable-speed rotational power, a forward/rearward-travel switch unit that operatively receives the combined rotational power outputted from the third component, and a transmission case that accommodates the planetary gear unit and the forward/rearward-travel switch unit (see, for example, Japanese Patent Unexamined Publication No. 2003-130174 (hereinafter, referred to as Patent Document 1)).
The traveling system transmission structure configured as described above can use a whole speed range of the variable-speed rotational power for each of forward and rearward variable speed ranges. Accordingly, this traveling system transmission structure is useful in expanding variable range of travel speed in which travel speed could be change in a non-stepwise manner without involving increase in size of a variable-speed rotational power outputting member, such as an electric motor or an HST, for outputting the variable-speed rotational power.
However, the conventional traveling system transmission structure described in Patent Document 1 has problems that the planetary gear unit is radially enlarged as well as that the planetary gear unit and the forward/rearward-travel switch unit are difficult to be assembled to the transmission case.
More specifically, the transmission case has a hollow peripheral wall part that configures an accommodating space for accommodating the planetary gear unit and the forward/rearward-travel switch unit, a rear support wall provided to the rear portion of the peripheral wall part, and a front support wall provided to the front portion of the peripheral wall part.
The forward/rearward-travel switch unit has a forward/rearward-travel input shaft that operatively receives the combined rotational power from the planetary gear unit, a forward/rearward-travel output shaft that outputs the rotational power to a travel unit, a forward/rearward-travel clutch mechanism that is capable of changing a rotational direction of the rotational power transmitted from the forward/rearward-travel input shaft to the forward/rearward-travel output shaft, and a clutch shaft that supports the forward/rearward-travel clutch mechanism.
The clutch shaft has a rear end supported by the rear support wall and a front end supported by the front support wall. The forward/rearward-travel clutch mechanism is supported by a rear portion of the clutch shaft that is positioned on a rear side, while the planetary gear unit is supported by a front portion of the clutch shaft that is positioned on a front side.
Specifically, the planetary gear unit has a hollow sun gear shaft provided with a sun gear that functions as the second component for receiving the variable-speed rotational power. The hollow sun gear shaft is externally inserted to the front portion of the clutch shaft, so that the planetary gear unit is accommodated in the accommodation space.
In other words, in the conventional traveling system transmission structure described in Patent Document 1, the clutch shaft, of which front end is supported by the front support wall and of which rear end is supported by the rear support wall, supports the planetary gear unit in addition to the forward/rearward-travel clutch mechanism.
In the configuration described above, the clutch shaft is required to have a large diameter in order to ensure enough strength thereof, which also requires increase in diameter of the sun gear shaft to be externally inserted to the clutch shaft.
The planetary gear unit needs be radially enlarged in order to obtain a predetermined speed reduction ratio with use of the sun gear shaft of a large diameter.
Further, in the conventional traveling system transmission structure described in Patent Document 1, the rear end of the clutch shaft is supported by the rear support wall that is formed integrally with the peripheral wall part, and the front end thereof is supported by the front support wall that is detachably connected to the front portion of the peripheral wall part. The forward/rearward-travel clutch mechanism and the planetary gear unit are supported by the clutch shaft that is supported by the rear support wall and the front support wall.
In the configuration described above, the forward/rearward-travel switch unit and the planetary gear unit need to be installed into the transmission case from the front end thereof, thereby resulting in another problem of deteriorated efficiency in assembling these units into the transmission case.
As disclosed in Japanese Patent Unexamined Publication No. 2003-276461 (hereinafter, referred to as Patent Document 2), there is another traveling system transmission structure including a planetary gear unit that has first and second components for receiving a constant-speed rotational power and a variable-speed rotational power respectively, as well as a third component for outputting a combined rotational power of the constant and variable-speed rotational powers, and a forward/rearward-travel switch unit that operatively receives the combined rotational power outputted from the third component, in which the constant-speed rotational power and the variable-speed rotational power are operatively transmitted from a pump shaft and a motor shaft of an HST respectively to the first and second components of the planetary gear unit.
In short, Patent Document 2 proposes the vehicle traveling system transmission structure that realizes an expanded variable range of travel speed in which travel speed could be changed in a non-stepwise manner by arranging the HST, the planetary gear unit and the forward/rearward-travel switch unit in series.
Specifically, the conventional traveling system transmission structure described in Patent Document 2 includes the HST that non-stepwisely changes a speed of the constant-speed rotational power, which has been operatively transmitted from a driving power source, into both normal and reverse directions and also outputs the changed variable-speed rotational power, the planetary gear unit that has the first component for operatively receiving the constant-speed rotational power from the driving power source, the second component for operatively receiving the variable-speed rotational power from the HST, and the third component for outputting a combined rotational power, the forward/rearward-travel switch unit that has a driving side for operatively receiving the rotational power from the third component and a driven side capable of outputting any one of forward and rearward rotational powers, and a speed-change operating member and a forward/rearward-travel switch operating member both of which can be manually operated.
The HST is configured so as to be shifted from a first maximum output state of outputting the rotational power that has a maximum rotational speed in one of the normal and reverse directions to a second maximum output state of outputting the rotational power that has a maximum rotational speed in the other of the normal and reverse directions. The planetary gear unit is shifted from a substantially non-output state to a maximum output state as the HST is shifted from the first maximum output state to the second maximum output state.
The traveling system transmission structure configured as described above can use a whole variable speed range of the HST that includes a normal rotation range and a reverse rotation range for each of the forward and rearward travel variable speed ranges, thereby being useful in expanding the variable range of travel speed in which the travel speed can be changed in a non-stepwise manner without involving increase of the HST in size. However, on the other hand, there arise problems that the HST and the planetary gear unit need to be accurately manufactured as well as to be controlled with higher accuracy in order to realize a vehicle stop state.
More specifically, in order to cause the vehicle to be stopped, the vehicle including the traveling system transmission structure configured so that the planetary gear unit is shifted from the substantially non-output state to the maximum output state as the HST that is shifted from the first maximum output state to the second maximum output state, it is necessary to accurately perform, with no variation, manufacture and assembly of the HST as well as establishment of the connection between the HST and the speed-change operating member such that the HST is accurately in the first maximum output state when the speed-change operating member is located at the initial position. Furthermore, in order that the planetary gear unit is in the non-output state when the HST is in the first maximum output state, it is necessary to accurately manufacture the respective gears of the planetary gear unit inclusive of a large number of components as well as to strictly control engagement among the respective gears.
It is possible to bring the planetary gear unit into the non-output state by repeatedly performing a cycle of electrically monitoring the non-output state of the planetary gear unit, determining with use of a controller whether or not the planetary gear unit is in the non-output state, and, in a case where the planetary gear unit is not in the non-output state, changing the output state of the HST so that the planetary gear unit is brought into the non-output state. However, repetition of such control cycles possibly causes hunting motion of the vehicle as well as requires highly accurate control.